Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, but single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they can produce polymers with improved physical properties.
Organometallic complexes that incorporate “indenoindolyl” ligands are known (see, e.g., U.S. Pat. Nos. 6,232,260 and 6,451,724). In many of the known complexes, an indenoindolyl group is bridged to another group, which may be a second indenoindolyl group. Some of the known bridged indenoindolyl complexes have constrained geometry or “open architecture” (see, e.g., U.S. Pat. No. 6,838,410). Indenoindolyl ligands are versatile because a wide variety of indanone and arylhydrazine precursors can be used to produce indenoindoles. Thus, substituent effects can be exploited and catalyst structure can be altered to produce improved polyolefins.
Finding the best way to activate catalyst systems based on indenoindolyl metal complexes, particularly supported complexes, is a continuing challenge. Routinely employed activators include alumoxanes (e.g., methyl alumoxane (MAO)), ionic borates (e.g., trityl tetrakis(pentafluorophenyl)borate (F20)), and alkylaluminum compounds (e.g., triethylaluminum, diethylaluminum chloride), while other more exotic activators such as aluminoboronates have also been used. (see U.S. Pat. No. 6,759,361).
Usually, a single activator is used, although mixtures of activators have been taught. Moreover, the support material is commonly treated with organoaluminum, organomagnesium, organoboron, or other reagents (see, e.g., U.S. Pat. No. 6,211,311), prior to combining it with the transition metal complex and any additional activator. Among the organoaluminum compounds often used to treat the support are alumoxanes and alkylaluminum compounds.
Not specifically taught is the idea of using multiple activation steps (not including the support treatment). For example, U.S. Pat. No. 6,559,251 teaches to combine an indenoindolyl titanium complex with triethylaluminum-treated silica, then activate the complex with F20 (see Example 4). In other examples, the catalyst is made by pre-treating silica with MAO, then combining the MAO-treated silica with a mixture of MAO and the transition metal complex. A subsequent activation step is not. used.
U.S. Pat. No. 6,583,242 teaches catalyst systems comprising MAO-treated silica, an indenoindolyl metal complex, and an activator. The activator can be MAO, ionic borates, or mixtures thereof. In a typical example, a solution containing MAO and the indenoindolyl metal complex is slowly added to stirred MAO-treated silica to provide a free-flowing powder catalyst system (i.e., an “incipient wetness” technique is used). A subsequent activation step is not used.
Of course, maximizing catalyst activity never goes out of style. Ideally, polyolefin manufacturers could achieve higher catalyst activities using the versatile indenoindolyl transition metal complexes without resorting to expensive or exotic activators. A valuable method would use conventional activators and would be easy to practice. A desirable method could provide catalyst systems with activities enhanced by an order of magnitude compared with known catalysts.